PhD student in spider behavioural ecology and evolution

Mantises eating mantises to make mantises

So last time was wining and dining with insect people. This time it’s back to dining *hur hur*.

*Hem*

To recap on sexual conflict: it happens whenever the optimal reproductive strategy (i.e. means of maximising one’s reproductive output) of one sex encroaches on that of the opposite sex and this happens because eggs are like elephants and sperm are more like ants – there are a helluva lot more of the latter but the former are a helluva lot bigger. (The analogy ends there. I wouldn’t want to suggest that eggs are scared of sperm like elephants and ants are…).

So while sexual conflict reflects a common inequality in gamete size/availability across many sexually reproducing species, the way that this conflict manifests varies significantly. One such way is through the development of clasping behaviours and structures in male water striders as I discussed in my first post with these structures helping males defend their ladies against any hopeful rakes. Another such way is through sexual cannibalism.

Would it be a surprise to anyone if I said that I was never met with glazed eyes whenever my research came up? Mild disgust, yes. Morbid fascination, sometimes. ‘Does that have any commercial applications?’, you’d be surprised. Most commonly, ‘Pardon?’ (Read: ‘I had only asked because I was being polite so I was only half listening but if I heard correctly then this conversation has taken a turn for the surprising. Let me charge your glass so you might tell me more of kinky insect noms’.)

This is what sexual cannibalism is according to the internet (with safe search on):

I don’t want to go into all the reasons sexual cannibalism makes such good nightmare fodder in erotic horror fiction and also don’t want to go into all the consequences that this interest has for the study of sexually cannibalistic species. There is an ongoing debate about this which has reached the internet already. I might go into detail one day but not now.

Instead the point I want to make is that the reason why sexual cannibalism is such an interesting behaviour in a biological context. The simple reason is that the sex enforcing this extreme behaviour are females rather than males. The previous example of male water striders clinging to resistant females is a much more common pattern while in the case of sexual cannibalism, females are very much the ones enforcing the costs.

It can be hard to reconcile how a behaviour which kills a member of one’s own species could possibly be maintained or even be developed in the first place and there are a number of conflicting hypotheses as to how cannibalism can be maintained. The main one I want to look at is called the ‘Adaptive foraging hypothesis’ which was central to my study of the springbok mantis, Miomantis caffra. This hypothesis suggests that sexual cannibalism is a foraging strategy employed by females in times of food limitation and it entails a subset of hypotheses about how this behaviour will manifest. (Aggressive spillover is another major contending set of hypotheses (2)).

To understand how sexually cannibalistic could prove an advantageous hunting strategy you have to remember how spiders and praying mantises live. Both species are cryptic sit-and-wait predators. Spiders generally hide in cracks or in corners while mantises are generally very well camouflaged to their environments and neither will tend to move unless disturbed. The advantage of this is that they can avoid detection by predators and potential prey. Hence they ‘sit-and-wait’. The downside of this strategy is that, if there aren’t many flies around that season, then there’s not a lot that they can do about it.

But female spiders and mantids have a way around this. Females of both species emit long distance sex pheromones from their bodies or, in the case of spiders, from their silk. These airborne chemicals (Chanel/Dior etc.) are detected by males who race to reach the female and mate with her. So, if food is scarce and females are particularly hungry, it only makes sense to ‘make use’ of the males who turn up of their own volition.

And a female consuming males due to low availability of food isn’t just about ensuring that the female doesn’t starve. It seems that female body condition (think of it as an arthropod BMI) can be strongly related to the number of eggs that female can produce aka her ‘fecundity’. She’s not just hungry. She’s not eating for two so much as 401.

Now there is a complication. There are two ways that sexual cannibalism can manifest and the difference is vitally important to understanding the costs and benefits of the behaviour. It’s all very technical but bear with me and we can take a break if it gets a little too complicated. Basically, sexual cannibalism can occur:

before sex…

or after (slash during).

Phew, made it. The former is called ‘non-‘ or ‘pre-copulatory cannibalism’ while the latter is called ‘post-copulatory’ or just ‘copulatory cannibalism’. Now, with post-cop cannibalism, females get to mate AND eat which raises the question of why any female would bother with pre-cop cannibalism. Furthermore, if a female eats every male that turns up without mating then she runs the risk of never getting the chance to mate before the seasons change (assume winter = death). So it’s a little hard to see why not all sexually cannibalistic species just eat the male after mating.

There a couple of potential reasons for this. Firstly, males might be in a better position to escape after mating. Secondly, mating can take a long, long time. Some mantises mate for around seven hours and female condition gets appreciably worse throughout the ordeal. Consequently, sometimes it’s a case of a meal now versus the chance of a meal later. However, the pressure to mate is high so it is expected under the Adaptive foraging hypothesis that females would be less likely to cannibalise males as time goes on and the prospect of a frosty death begins to loom.

Pros and cons of cannibalism without mating (according to the Adaptive foraging hypothesis)

Basically, there are two major primary ecological constraints which can support the development of sexual cannibalism: food availability and mate availability. If you have a small number of food species around but a surplus of potential suitors, sexual cannibalism is a valuable asset for a female. In my own work I mainly looked at the availability of food and how that influenced the frequency of sexual cannibalism. In the wild this is generally the result of major environmental changes which influence the availability of prey. In the lab it involves with-holding locusts from a lab population. Science!

Next time: I need to take a break from mantises. I’m sure an equally dignified topic will present itself.

References:

The Adaptive foraging hypothesis was formally described by Newman and Elgar in 1991 and entails five different hypotheses. Based on work with the fishing spider Dolomedes sp.

Dolomedes dawwwww

The Aggressive spillover hypothesis was generated when Arnqvist and Henriksson found that the Adaptive foraging hypothesis didn’t fit with their own experimental spider species. It suggests that the level of female aggression is determined during development and that aggressive female nymphs (young) are more likely to survive into adulthood as they’re more voracious feeders. As adults they continue to be aggressive to anything that comes near them, including males. Under this hypothesis, sexual cannibalism is maladaptive i.e. not advantageous and is merely the by-product of the survival of more aggressive young.